When I have it active going downhill, letting off the throttle I do drop speed and I see the boost being generated, but how do I maintain my speed?
2018 Owner's Manual guidance directly gives the answer: Pg 226
Note: If your vehicle has Adaptive Cruise
Control, the speed control system uses the
selected engine brake mode, transmission
gears, and brakes to maintain the set speed
and distance to the vehicle being followed.
Automatic Engine Braking
1. Press the button twice to switch on
automatic engine braking.
2. Release the accelerator or brake pedal.
The system remembers your vehicle
Note: This feature offers smoother, less
aggressive engine braking during downhill
descents. It may not apply braking if the
vehicle speed is not increasing, although it
will apply full engine braking force when
needed to prevent acceleration. The system
automatically activates the engine brake
and, if necessary, downshifts the
transmission to lower gears to not exceed
your vehicle's set speed when the brake or
accelerator were last released.